This application claims priority of the German patent application 202 01 592.0, filed on Feb. 2, 2002, which is incorporated by reference herein.
The innovation relates to an optical lens arrangement for rangefinder systems in photographic cameras with fixed viewfinder magnification SK and fixed measuring base MB.
Rangefinder systems in photographic cameras are used to select the field of view that can be photographed with the respective camera lens and, at the same time, to focus the camera lens sharply onto the object lying in the field of view. When used with interchangeable camera lenses having different focal lengths, the rangefinder system contains different masks as bright-line frames, which are assigned to the field of view for the respective focal length and which are reflected into the viewfinder optical path.
For focus adjustment, the rangefinder system contains two observation optical paths, which are separated from one another in the camera housing by a fixed mechanical base length. The viewfinder optical path has a fixed observation direction, while the observation direction of the measuring optical path can be changed by means of an optical element coupled with the focus drive of the camera lens. In the rangefinder eyepiece, the images from both observation directions are superimposed. The focus adjustment is achieved when the images are congruent.
The accuracy of thie focus adjustment depends on the geometrical length of the measuring base and on the magnification of the viewfinder system, with the viewfinder magnification affecting the survey of the bright-line frames for different focal lengths of the camera lens in the field of view. A small viewfinder magnification makes it expedient to represent bright-line frames for short focal lengths to medium focal lengths. A large viewfinder magnification makes it possible to represent bright-line frames for medium to long focal lengths. In this case, however, it is true that the image section represented by the bright-line frames decreases greatly in size from shorter to longer focal lengths. Both the image definition of the image section and the focus adjustment for objects in the respectively smallest bright-line frame are made significantly more difficult because of this.
In practice, photographic cameras have therefore been equipped with rangefinder systems having a different viewfinder magnification. The user can then select the camera with the appropriate viewfinder magnification corresponding to the camera-lens focal lengths that he or she uses most of the time. If he or she wishes to use focal lengths lying outside the suitable range with this camera, then he or she needs to make a compromise between representing the full photographing field of view or the accuracy of the focus adjustment.
As compensation for a compromise in the short focal-length range, special fit-on viewfinders have therefore been developed, which are fitted into the flash shoe of the camera and which ensure exact selection of the full photographing field of view. For focus adjustment, however, it is then necessary to change the eyepiece (Handbook of the Leica System, 1996/97, page 5-18, brilliant view finder for 21 mm and 28 mm lenses).
As compensation for a compromise in the long focal-length range, special viewfinder front attachments have been developed, which are coupled with the longer focal-length camera lens and in front of which both view windows of the rangefinder system are switched (Handbook of the Leica System, 1996/97, page 5-16, view finder front attachment for 135 mm lens). The field of view assigned to a camera lens with 135 mm focal length is magnified by the viewfinder front attachment so much that it fills up the bright-line frame of a camera lens having, e.g., 90 mm focal length. The displayed bright-light frame does not therefore match with the focal length of the camera lens. The complexity of construction for the viewfinder front attachment is considerable.
Viewfinder magnifications used in practice are SK=0.58/0.72/0.85. The geometrical length of the measuring base is, for example, equal to 69.25 mm, so that an effective measuring base of 40.2 mm/49.9 mm/58.9 mm is obtained. When the effective measuring base is greater, the focus adjustment can be visually assessed in a commensurately more accurate way. This is especially important, in particular, for camera lenses with a high aperture and therefore a small depth of focus.
It is therefore an object of the innovation to provide a way in which the predefined viewfinder magnification can be increased in a simple way when required.
This object is achieved according to the invention by providing for an optical lens arrangement for a rangefinder system in a photographic camera with a fixed viewfinder magnification SK and a fixed measuring base MB, wherein the lens arrangement is configured to be coupled with a rangefinder eyepiece of the rangefinder system and is designed as an afocal telescope with a minimum 1.1 times and a maximum 1.7 times magnification SL.
The fact that the lens arrangement is fitted on the rangefinder eyepiece means that only a single accessory arrangement is needed, which magnifies the images from both superimposed rangefinder optical paths together. This configuration as an afocal telescope does not affect the imaging condition for the two beams coming from the rangefinder system. In particular, observation can furthermore be carried out by an individual wearing spectacles. The specified magnification range makes it possible, with the predetermined viewfinder magnifications used in practice, for the effective measuring base to be magnified to the value of the predetermined geometrical measuring base. This approximately 25% magnification of the effective measuring base leads to a corresponding increase in the adjustment accuracy. The clearly visible larger viewfinder image provides significantly improved perception of feature details inside the respective bright-line frame, especially in the case of telephoto lenses.
It is advantageous for the magnification SL of the lens arrangement to be selected in such a way that the product of the viewfinder magnification SK of the camera and the magnification SL is at least very nearly equal to 1. The size of the viewfinder image then corresponds to the scene observed with the naked eye, so that convenient binocular observation of the feature to be photographed is possible. For example, the viewfinder magnification SK may be approximately 0.85 and the magnification SL may be approximately 1.25, or the viewfinder magnification SK may be approximately 0.72 and the magnification SL may be approximately 1.4, or the viewfinder magnification SK may be approximately 0.58 and the magnification SL may be approximately 1.7.
With a construction of the lens arrangement as a two-lens Galilean telescope, it is possible to ensure reproduction quality at the level of the built-in rangefinder system. A Galilean telescope is distinguished by an erect image and by small overall length. However, the viewing field is limited because of the internally located exit pupil, but this does not represent a disadvantage at the small magnifications required here. Preferably, the focal length f1 of the positive objective component satisfies 15 mm less than f1 less than 40 mm and the focal length f2 of the negative ocular component satisfies: xe2x88x9235 mm less than f2 less than xe2x88x9212 mm.
It may also advantageously be possible for the lens arrangement to be coupled on the eyepiece side with a dioptric compensation lens. An external screw thread M12xc3x970.5 on the front side and an equivalent internal screw thread on the eyepiece side may be provided as coupling means.